1. Field of the Invention
The present invention relates to a carry circuit and more particularly to a carry circuit used in a digital processing unit which can execute a multi-bit adding operation.
2. Description of the Prior Art
It is well known that, in an adding operation of a digital processing unit, it is very important to perform the processing of a carry signal, indicating an overflow of addition, at a high operating speed. In a parallel multi-bit addition, the existence of the carry signal at one bit section depends on a result of addition at this bit section and the existence of the carry signal from the previous stage.
A well known ripple carry method is the most basic method to process the carry signal, but it is difficult to perform addition at high speed using this method. Particularly, in the addition of a long bit string, e.g., 32 bits or 64 bits, the ripple carry method is unsuitable.
To resolve this problem, a carry look ahead (hereafter called "CLA") method has been proposed. This method is useful in a high-speed adding operation, since the processing of the addition and the processing of the carry signal can be independently performed.
In the CLA method, the carry signal is produced in accordance with a combination of a pair of bits to be added. That is, if both bits to be added are "1", the carry signal necessarily appears. Further, when either one of the bits to be added is "1" and when a carry signal from the previous stage exists, the carry signal also appears. Except for the above-mentioned combinations, no carry signal appears. To realize this method, a decoding logic (e.g., a programmable logic array) may be used in which two bits to be added are applied to respective inputs, and a carry signal is derived from an output in accordance with a combination of these two input bits.
By using the CLA method, a carry signal at each bit section is produced at a high speed, because existence of the carry signal can be determined independently of the result of addition. However, propagation of the produced carry signal from the lower bit portion to the higher bit section is necessary. The propagation of the carry signal is performed through a carry signal line, and therefore the delay of signal propagation is a big problem in the CLA method. This delay has a great influence on an adding operation of a long bit string.